The invention relates to a powdered metal filter composition and processes for producing the same for the generation of filtered packs particularly useful for the filtration of liquid polymer in the manufacture of synthetic fibers. The filtration of the liquid polymers is well known in the art as is more completely described in U.S. Pat. No. 3,896,028.
In the past, a number of different filter media have been employed including silica sand, metal shavings as well as metal powders. Until the advent of the present invention, silica sand was the preferred filter media.
Silica sand exhibits a number of disadvantages. It is extremely brittle with the result that the development of fines of silica sand tends to obstruct the orifices in the fiber spinnerette. Additionally, the specific surface area of silica sand is substantially less than that atomized metal powder for any given filter pack volume. Additionally, silica sand tends to break down or fracture under pressure in the filter bed rather than deforming thereby favoring the production of fines during use.
Metal shavings exhibit less filtration efficiency than either silica sand or metal powder because of the relative lack of surface irregularities. Moreover, they tend to deform or break in use.
Previously, metal powder was sintered into a filter form for convenience in handling. However, this tended to reduce filtration efficiency by reducing the specific surface area available for filtration. Moreover, the process of pressing and sintering was extremely costly. This cost factor was compounded by the fact that the rapid loss of filtration efficiency by clogging of the interstices required reprocessing by burning out the polymeric sludge which in turn tended to further reduce filtration efficiency. The result was that the sintered metal powder filter frequently had to be discarded with as little as four uses.
Atomized metal powder which is prepared under specific conditions exhibits an extremely irregular surface which tends to enhance its filtration efficiency. However, previous metal powder filters prepared from loose stainless steel powder lacked the mechanical strength under pressure to maintain their apparent density within those limits for optimum filtration efficiency. Hence, the previous resort to sintering the metal powder to achieve some degree of mechanical strength.
It is among the objects and advantages of the present invention to produce a powdered metal filter composition which exhibits both the apparent density and enhanced resistance to compressibility for optimum filtration efficiency with little or no particle deformation and collapse under operating pressures. These characteristics have been achieved by increasing the percent of silicon, manganese and carbon in the melt feed for the atomization process as well as by controlling the angle of intersection of the water atomization jets with the molten metal stream.
The chemistry of the composition disclosed and claimed herein increases its mechanical strength without embrittling or adversely affecting the development of irregular surface shape during atomization.